There is one giant glaring error with that model, and I don't know that anything is being done about it. (Disclaimer: I've never used Pandora, but I have used last.fm).

Sometimes, I want to listen to classic rock. Sometimes metal. Sometimes trance. Sometimes, I want to listen to Final Fantasy soundtracks. I do not want to listen to all of them at once. I do not want to listen to some strange meld of them all. Short of making a half dozen usernames for each genre I like, what is the solution? Obviously just getting people to tell you what they like doesn't work - "classic rock" covers a large area, and not the same one in everyone's minds.